1. Technical Field
The present invention relates to a fixing device, and more particularly, to a fixing device for use in an image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of these features.
2. Background Art
In electrophotographic image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of these features, an image is formed by attracting developer or toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process is followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium with heat and pressure.
In general, a fixing device employed in electrophotographic image formation includes a pair of generally cylindrical looped belts or rollers, one being heated for fusing toner (“fuser member”) and the other being pressed against the heated one (“pressure member”), which together form a heated area of contact called a fixing nip. As a recording medium bearing a toner image thereupon enters the fixing nip, heat from the fuser member causes the toner particles to fuse and melt, while pressure between the fuser and pressure members causes the molten toner to set onto the recording medium.
Various methods have been proposed to provide a fast, reliable fixing process that can process a toner image with short warm-up time and first-print time without causing image defects even at high processing speeds.
For example, a known belt-based fixing device employs an endless flexible belt looped into a generally cylindrical configuration extending in an axial direction thereof for rotation in a rotational, circumferential direction thereof. In this fixing device, a stationary fuser pad is disposed inside the loop of the belt, with a pressure roller disposed parallel to the belt to press against the fuser pad via the belt to form a fixing nip therebetween. A generally flat, reinforcing plate is provided, having its narrow face in contact with the fuser pad to reinforce the fuser pad against nip pressure. The belt assembly is provided with a low-friction sheet of lubricant-impregnated material that supplies lubricant between the stationary pad and the belt.
According to this method, the fuser belt is equipped with a tubular holder of thermally conductive metal, or heat pipe, disposed inside the loop of the fuser belt for heating the fuser belt through conduction. A heater is disposed inside the heat pipe, from which heat is imparted to the entire circumference of the fuser belt looped around the heat pipe. The heat pipe has a longitudinal side slot defined on one side thereof, within which the fuser pad is accommodated. Provision of the slotted heat pipe thus enables the fuser pad to maintain its proper operational position while subjected to external forces during operation.
The inventors have recognized that one problem associated with the belt-based fixing device is that the lubrication mechanism, provided between the stationary pad and the belt, prematurely fails to work over time. Premature failure of the lubrication mechanism may occur, for example, due to variations in nip pressure during operation causing the lubricant to flow from where the pressure is relatively high to where the pressure is relatively low along the low-friction sheet, resulting in a localized loss of lubrication where the nip pressure is highest across the fuser pad. Not surprisingly, lubrication failure in the fuser belt assembly entails various adverse consequences, including accelerated degradation due to abrasion of the fuser pad and the belt at the fixing nip.